An earthquake is a sudden shaking of the Earth’s surface resulting from the rapid release of stored energy in the planet’s crust. This energy travels outward as seismic waves, causing the ground to tremble. Scientists classify these events based on the underlying geological or mechanical source mechanism to understand how this energy is discharged.
Earthquakes Caused by Plate Movement
The vast majority of the world’s earthquakes, including the most powerful events, are tectonic in origin. They occur because the Earth’s rigid outer layer, the lithosphere, is broken into large segments called tectonic plates that constantly move relative to one another. This slow motion creates immense stress and strain along plate boundaries and faults, which are fractures in the crust.
When the stress exceeds the rock’s strength, the crust suddenly ruptures and slips, releasing the accumulated elastic energy that has been stored like a stretched rubber band. This sudden displacement along the fault plane generates the seismic waves felt as an earthquake. The type of fault movement is directly related to how the tectonic plates are interacting, categorized into three primary modes.
At divergent boundaries, plates pull apart, stretching the crust by tensional forces and leading to movement along normal faults. In a normal fault, the rock above the fault plane, known as the hanging wall, moves downward relative to the footwall. Earthquakes in these zones, such as the Mid-Atlantic Ridge, are generally smaller, typically registering below magnitude 7.0, because the crust is being stretched and thinned.
Conversely, at convergent boundaries, plates collide, creating compressional forces that result in reverse or thrust faults. The hanging wall is pushed up and over the footwall, shortening the crust. This collision is responsible for the largest seismic events, known as megathrust earthquakes, which occur in subduction zones. These events release tremendous amounts of energy, producing the highest recorded magnitudes, such as the magnitude 9.5 event in Chile in 1960.
The third type is the strike-slip fault, characteristic of transform boundaries where plates slide horizontally past one another due to shearing forces. The San Andreas Fault in California is a prominent example of this movement, where the ground shifts laterally with little to no vertical motion. Strike-slip faults can produce major earthquakes, with the largest capable of reaching a magnitude of approximately 8.0.
Earthquakes Related to Magma Activity
A second category of earthquakes is associated with active volcanic systems, often called volcano-tectonic events. These are caused by the localized movement of magma and associated fluids beneath a volcano, not by large-scale plate interactions. As magma rises into subsurface reservoirs, it forcefully pushes against the surrounding rock.
This pressurization creates intense local stress that fractures the brittle rock, generating distinct seismic signals. These events are often shallow, typically occurring at depths less than 10 kilometers. They frequently appear as a seismic swarm—a rapid increase in small quakes without a clear main shock.
The movement of hydrothermal fluids (hot, pressurized water and gas) also plays a significant role in generating these earthquakes. This fluid movement and resulting pressure changes cause a continuous seismic noise known as volcanic tremor, which can be a reliable precursor to an eruption. Monitoring these localized earthquakes is a primary tool for volcanologists, as an increase in frequency or a change in depth can signal an imminent eruption.
Earthquakes Resulting from Underground Collapse
The final category, collapse earthquakes, represents the smallest and most localized form of seismic activity. These events are caused entirely by the failure of a subterranean cavity, unrelated to tectonic stress or magma pressure. They occur most often in regions underlain by soluble rock, such as limestone, where groundwater dissolves the rock to form karst formations.
When the roof of one of these natural voids or an abandoned mine shaft fails, the sudden collapse of the overlying rock mass releases seismic energy. The event is driven by gravity, as the rock falls into the open space below.
Because the energy release is limited by the size of the cavity, these earthquakes are shallow and rarely register above magnitude 3.0. The resulting ground shaking is felt only directly above the collapse site, sometimes causing localized structural damage but posing little regional threat.